We live in an era - the Anthropocene - where humans and societies are reshaping and changing ecosystems. Pollution, human-made climate change and overfishing have all altered marine life and ocean food webs.

Methylmercury builds up in the muscle tissue of fish across the food web, "bioaccumulating" in larger and high trophic level predators. This is why larger pelagic fish (for example, tuna, marlins, billfishes and sharks) - those that eat a lot of fish - are in general considered riskier to eat than smaller ones.

Overfishing can also exacerbate the mercury levels in some fish species. Pacific salmon, squid and forage fish, as well as Atlantic bluefin tuna and Atlantic cod and other fish species are susceptible to increases in methylmercury due to rising ocean temperatures.

Our modelling research work shows that Chinook salmon, the largest Pacific salmon species and main prey of endangered southern resident killer whales, is projected to be exposed to high methylmercury accumulation due to changes in its prey that are driven by climate change.

Under a worst-case climate-change scenario, where greenhouse gas emissions continue to increase and global temperatures reach between 2.6C and 4.8C by 2100, Chinook salmon will see a 10 per cent increase in methylmercury. But under a best-case scenario, where emissions are low and global temperature rise is in the order of 0.3C to 1.7C at the end of the century, mercury levels would increase by only one per cent.

For forage fish, such as Pacific sardine, anchovy and Pacific herring, which are key ecological and commercial species in the Pacific Rim ecosystem, the methylmercury increase is projected to be 14 per cent under the influence of high emissions and three per cent under low emissions. Here again, this increase is driven by dietary shifts and changes in the food web composition due to warmer oceans.

When one species is overfished, fishing fleets expand and adjust their targets, often fishing down the marine food webs. The cascading effects lead to changes in prey and foodweb composition for the remaining species, likely altering the transfer of organic contaminants such as persistent organic pollutants and methylmercury in top predators.

When fish are removed from the food web, larger fish and top predators may be forced to consume more or different prey, or smaller fish than they usually do. These fish can be highly contaminated with mercury.

Based on this evidence, the public health community should revisit and revise fish consumption guidelines for those who are most likely to be exposed to mercury (coastal communities) or experience negative effects (pregnant women, infants and children).

Our simulations show that the projected methylmercury concentrations in forage fish and Chinook salmon will surpass Canada's mercury consumption limits this century, as well as the consumption advisory level issued by the World Health Organization.